In some cases it may be necessary not only to adjust the angle or the alignment of a vehicle exterior mirror, but also its distance from the vehicle body. Such a length adjustment may, for example, be necessary in a commercial vehicle with an extra-wide trailer to offer the driver an adequate view to the rear. If no extra-wide trailer is to be pulled, the vehicle exterior mirror which is protruding far outwards can be brought nearer to the body via the length adjustment, which not only has aerodynamic advantages, but also the risk of damaging the exterior mirror when parking or driving through narrow passages decreases.
From the publication DE 39 38 961 A1, a telescoping adjustment for an exterior mirror is known. In this case, the exterior mirror is attached to a continuously adjustable telescopic tube, which in turn is articulated by means of two flat profiles of a holding frame on the vehicle cabin. The length adjustment of the exterior mirror takes place via a positioning motor which drives in a corresponding direction a threaded spindle which is operatively connected to a tube of the telescopic tube assembly.
Since the mirror is constantly exposed to the elements, it may happen that the telescopic tube assembly becomes jammed due to dirt or dust, icing-up, rust, etc., and can no longer be adjusted. When the driver operates the electric motor to move the exterior mirror in this case, the motor due to the adjustment mechanism being blocked, may be damaged.
The same applies if the driver does not stop the electric motor in time before the telescopic tube reaches its respective adjustment end-position.
Furthermore, it has been found that the threaded spindle and the telescopic tube can easily become jammed when, during the extension or retraction of the telescopic tube, the threaded spindle drives the telescopic tube against a stop. When the telescopic tube is subsequently to be moved again in the opposite direction, the drive torque of the electric actuating unit cannot alone release the jammed parts.
Different clutch devices are known from prior art. In the case of the above mentioned exterior mirror adjustment means for a utility vehicle, a simple, economical, low maintenance and robust solution is required. It has been found in practice that none of the known clutches is suitable for this purpose. For reasons of overload protection, non-switchable clutches are excluded. Externally controlled clutches are too costly and also not practical. Against this background, an automatically switching clutch is necessary which overcomes the problems of the prior art.